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(Disclaimer - following answer is from a brand new ref)
As mentioned, the rule book has the applicable diagram. In summary, if it's in the lane (and the semicircle a FT shooter occupies) or in the area defined by the outside points of the free throw lane and the corners, the ball is inbounded at the nearest point on the endline (but not between the lane boundaries). If outside of this area, it's at the nearest point of the sideline.